The present invention relates to a scanning method and apparatus applicable to variable magnification copying machines.
Recently, a marked tendency to increase the copying speed of electrophotographic copying machines and to provide them with various additional functions is seen. One of the additional functions is to vary the magnification of copy size by enlarging or reducing copy size. In particular, it is a more general tendency to provide electrophotographic copying machines with a function of reducing copy size.
Therefore, the conventional copy size reduction systems will be explained first for a better understanding of the invention.
The conventional copy size reduction systems can be roughly classified into the following four types: (1) Overall exposure reduction system, (2) Slit exposure reduction system of original document movable type which is applicable to sheet originals, (3) Slit exposure reduction system of original document platen movable type, and (4) Slit exposure reduction system of original document stationary type.
Of the above-mentioned four types of copy size reduction systems, the slit exposure reduction system of original document stationary type (4) is as good as the overall exposure system (1) in the point that the quality of copies largely not effected by the kind and weight of original document to be copied. Additionally, the slit exposure reduction system (4) is excellent in that the system is not to be oversized unlike the overall exposure reduction system (1), but it requires that the moving speed of the optical system and the illumination system (hereinafter collectively referred to as the optical system) have to be varied depending upon a selected copy magnification. Generally, the moving speed of the photoconductor is constant regardless of the copy magnification. Therefore, when the moving speed of the optical system at unit magnification, which is also identical to the moving speed of the photoconductor, is V.sub.F, and a selected magnification is m (m&lt;1 at reduction copy size), the moving speed of the optical system at the exposure step, .upsilon..sub.F, is determined by EQU .upsilon..sub.F =(1/m)V.sub.F ( 1)
Taking into consideration that the moving speed of the optical system at the exposure step has to be varied depending upon a selected magnification, a copying machine employing the slit exposure reduction system of original document stationary type will now be discussed a little further. In this type of copying machine, generally more copies are made with unit magnifications than with other magnification and in order to maximize the number of copies per unit time with unit magnification, with respect to the moving speed of the optical system during exposure, a limit design has to be made on the basis of the moving speed of the optical system at unit magnification. However, at the time of reduction of copy, the moving speed of the optical system at the exposure step has to be set greater than that at unit magnification. This is, however, difficult in view of the life of the lamps in the illumination system and noise and other limitations so that the increase of copy speed and improvement of variable magnification function are difficult to attain at the same time.
Practically, the moving speed of the optical system during exposure step at unit magnification is determined by taking into consideration the moving speed of the optical system at reduction of copy size, so that the copy speed at unit magnification which is originally possible to be increased is sacrificed.
In the slit exposure reduction system of original document platen movable type (3), the moving speed of the original document platen during exposure at the time of copy reduction has to be increased in comparison with the moving speed of the original document platen at unit magnification for the same reason as mentioned above. Therefore, there may be a risk that the original document, in particular, a book type original document is maybe shifted from its original position on the original platen by the change of the speed of the original document platen during the exposure step and its return step so that the copy image is shifted. Therefore, the slit exposure reduction system (3) is not always suitable for high speed copying and also has problems similar to those of the slit exposure reduction system (4).